Just as businesses have begun to grudgingly accept OS X, sales are slipping in what may be the Mac's long, slow good-bye

For the last five years, Mac sales have grown faster than PC sales, and even as PC sales declined in the last several years, Mac sales kept growing. They reached between 10.5 and 11.8 percent of U.S. PC sales in 2012, depending on whether you believe IDC or Gartner. That's the highest proportion Apple's Mac has achieved for several decades and a remarkable return from its death's-door days in the late 1990s.

But something flipped this year: Apple reported this week that Mac profits are down 7 percent versus a year ago. Mac sales slid 11.2 percent and PC sales were flat (if you believe IDC) or Mac sales dropped 2.3 percent as PC sales rose 3.5 percent (if you believe Gartner). Either way, Macs are no longer growing faster than PCs, and in fact PCs are now outpacing Macs. The United States has long been the Mac's biggest stronghold, and Mac sales data from the rest of the world is rarely reported by the analyst firms. We don't know how Apple is really doing when it comes to Mac sales elsewhere, but we know it's negligible in most countries.

It's ironic that the Mac may have reached its peak just as it's gained at least grudging acceptance in corporate environments. Companies like Cisco Systems now support Macs as equal citizens, and it's common to see lots of MacBook Airs at CIO conferences, not just at design and developer conferences (creatives and developers have long been Mac bastions).

The new OS X Mavericks has beefed up its security and management features, adopting most of those in iOS 7, as well as supporting Apple's new enterprise licensing mechanism. Many of Apple's Mac models have also been revved to use Intel's new "Haswell" chip. Macs are not just power systems, but more manageable by IT than ever using now-familiar mobile management servers.

But the timing appears off. There was a wave of Mac adoption for the last five years that had several contributors: the disappointment in Windows Vista, the rise of the iPhone, and the rise of the iPad. iPad adoption was probably the biggest factor; it's responsible for the strong decline in PC sales as well. The iPad "sold" the Mac to recent converts, and it absorbed PC dollars from the large "non-religious" user community.

Today, iPad sales are leveling off -- a large percentage of homes have at least one, and cheap Android tablets are filling the same demand for lower-income households and countries. Tablet sales to businesses have also flattened, notes 451 Research analyst Chris Hazelton, now that those business users that benefit from using iPads have them.

Meanwhile, PC sales appear to be up mainly due to business sales, a consequence of all those PCs running Windows XP finally being converted to Windows 7, which often means getting a new PC. Consumer sales appear to be flat, probably because of persistent disquiet over Windows 8 -- most Windows users I know don't want it and either hang on to their older PCs, convert to Mac (though most of that stripe did so last year) or find a small-business model that still comes with Windows 7. In other words, what we're seeing in the Windows world is what Wall Street calls a dead-cat bounce, not a revitalization of the PC market.

And now, even with new Mac models and the continued disappointment of Windows 8.1, we may have reached peak Mac. Tablets are clearly Apple's future computing platform, as CEO Tim Cook said yesterday in the company's quarterly earnings call, citing the 64-bit A7 processor that debuted in the iPhone 5s and forthcoming iPad Air. A Mac lasts for years, and Apple has kept new versions of OS X compatible with most five-year-old models; the pressure to upgrade to a new version likely won't be strong for the foreseeable future.